nullableValueMap
Turns a Map into a map with a nullable value type.
Intended to be used in conjunction with platform types such as Map<Int!,
String!`>! or in other words, when you deal with Java and you want to turn a platform type into a nullable type.
Basically it is a replacement for cast but without introducing one explicitly (we only give an additional hint to the compiler that we expect a nullable type and not a non-nullable one). For instance, instead of writing getPersons() as Map<String, Person?>
you can write nullableValueMap(getPersons())